1. Field of the invention
The present invention relates to chucks for gripping workpieces and, more particularly, to a quick-change collet chuck for gripping and installing container caps during an automated high-volume filling and capping process.
2. Description of the Background
The filling and capping process generally entails supplying containers along a conveyor, automatically filling them at a filling station, and automatically capping them at a capping station. Various testing and control functions may be performed along the way, e.g., testing and control of fill volume, cap torque, conveyor velocity, etc. The apparatus which performs the process must be capable of accommodating a wide variety of containers and caps (both caps and containers may vary in size and shape), and this is accomplished by a universal chuck which allows quick and easy grasping and manipulation of different cap sizes.
Current common methodology for screw cap positioning and torquing include the following types of chucks:
Tapered Chuck PA1 Friction Disk Chuck PA1 Donut Chuck PA1 Segmented Chuck
The action of the Tapered Chuck and the Friction Disk chucks is to apply axial (downward) force to generate the friction drive between the cap top (rim) and the chuck taper or friction disk. Having to avoid damage to the container, cap, and/or thread the axial force is limited. Thus tapered chucks and friction disc chucks can only handle a limited type of caps in relatively low torque applications which severely restricts the usage of these chucks. Another disadvantage of these chucks is their possible contamination by their own (or from the caps) particulates. Shavings may prevent required torque transfer since the driving axial force is limited in order not to damage the container and/or thread. These shavings can cause slippage that will perpetuate the problem. Simple friction drives such as the Tapered Chuck and the Friction Disk Chuck are not desirable in pharmaceutical clean packaging environments due to potential particulate generation from slippage.
The Donut Chuck includes a urethane ring (open center diameter matched in size to the cap) that, when a concentric cylinder is actuated, swells inward to clamp (radial pressure) on the outside of the cap thus enabling torque transfer as required. This is a friction drive but the friction force is generated not by axial but by a radial force compressing the cap. Normally this allows for a significantly higher torque range compared to the simple friction drives mentioned above. No axial force to risk the damage of the container or threads. However, there are a significant number of parts to be changed when changing from one cap size to another, enough that often the complete chuck is exchanged. Besides the tool requirement to do this, there is significant cost involved. The Donut Chuck has a working torque range good for up to medium (average) torque requirements (The particulate generation is minimal due to the clamping force being well in excess to what is required for transfer of the normal required torque thus resulting in no slippage between the cap and the donut).
The Segmented Chuck concept is specifically for the high torque range caps that typically have more severe serrations or other significant protrusions on the outside cap (radial) surface. The segmented chuck may, for instance, include a 3 piece segmented chuck jaw set (each segment occupying 120 degrees). However, this 3 piece segmented design is very heavy and clumsy, and it suffers from somewhat unstable jaw segments. In addition, the multi-segmented jaw set concept is very expensive to manufacture, and it does not lend itself to quick changing for different size caps. The chuck jaws are designed to match the cap outside profile and by a true interlock (when the jaws close) to facilitate positive (i.e. non-friction) drive for high torque requirements. Due to the expensive segmented die jaws concept of positive locking being very different from the Donut Chuck friction concept, this chuck does not lend itself economically to any simple applications. Again the change parts are enough trouble that for each cap size a complete new chuck is the practical solution. A whole chuck (inclusive of jaws) needs to be changed. Moreover, the mass of the three piece segmented chuck results in a high inertia which interferes with high speed operation. The chuck is servo driven and the servo motor provides positive feedback on the power required to turn/torque the cap. The high inertia of the chuck contaminates this data and limits the torquing speeds (and the overall production rates). A lower inertia results in more accurate torquing and higher production speeds.
It would be a great advantage to have a quick-change low-inertia collet-type chuck to allow quick and effortless swapping out of different size jaw sets for different size caps.
The Collet Chuck concept of a universal chuck to actuate a quick-change one-part collet as the only change part is far superior to the Donut Chuck and the Segmented Jaw Chuck since collets can be made to work the whole working range of these other two chucks. A low cost urethane lined collet will drive the caps with lower torque requirements. A machined contact profile collet will drive the caps with high torque requirements (positive interlocking with the external cap profile). Even asymmetric caps could be clamped in custom collets without requiring a special chuck change (The collet orientation relative to the chuck is always an exact repeat and servo drive allows an exact chuck orientation repeat). Preferably, there should be virtually no down time (or skill level) associated with the collet change.